1. Field of the Invention
The present invention relates to a liquid crystal display device which is excellent in light utilizing efficiency and which provides a brilliant display easy to see.
The present application is based on Japanese Patent Application No. Hei. 11-135334, which is incorporated herein by reference.
2. Description of the Related Art
There is investigation of a liquid-crystal display device which is used both in a reflection and a transmission mode so that the device can be visually recognized in use of the transmission type liquid-crystal display device in a dark place, or the like, by addition of an illuminator while the merit of a reflection type liquid-crystal display device of small power consumption is used widely. For example, a liquid-crystal display device using a semi-transmission type reflection plate, or the like, has been proposed. The system utilizing such a semi-transmission type reflection plate, however, had a disadvantage that the system in either mode was inferior in brightness to the reflection or transmission type device for exclusive use because light was separated into reflected and transmitted light by a half-mirror effect.
In consideration of the above description, there was a proposal for performing the improvement by using a reflection polarizer in which the sum of reflectivity and transmissivity was able to exceed 100%. The plane of vibration of linearly polarized light was, however, changed (by 90 degrees) between reflection and transmission, so that display was inverted. Hence, when a light was switched on in the evening twilight to make visual recognition in a transmission mode, display based on a reflection mode emphasized contrast so that the display became hard to see in either mode in the evening twilight. Moreover, the display became dark in a transmission mode because of absorption by a light absorber disposed for preventing a black display from being conspicuous. There was a problem that lowering of blackness due to the conspicuousness made contrast low if the aforementioned disadvantage was to be corrected.
An object of the present invention is to develop a liquid-crystal display device of a good visual recognition property which is excellent in brilliance both in a reflection and a transmission mode and in which inversion of display does not occur.
According to the present invention, there is provided with a liquid-crystal display device comprising: a light pipe including light output means formed on an upper surface of the light pipe; a light source disposed near to an incident side surface of the light pipe so that incident light from the light source is output from a lower surface of the light pipe through the light output means; a reflection polarizer disposed on the lower surface of the light pipe for supplying linearly polarized light obtained by reflecting natural light; and a liquid-crystal shutter disposed above the upper surface of the light pipe, the liquid-crystal shutter including liquid crystal cells, and at least one polarizing plate.
According to the present invention, a structure in which a light pipe is disposed between a reflection polarizer and liquid-crystal cells is provided so that linearly polarized light supplied through reflection by the reflection polarizer can be directly utilized for liquid-crystal display. Absorption loss due to the polarizing plate on the visual recognition back surface can be prevented. Lowering of light utilizing efficiency in a reflection mode is slight to be no more than absorption loss, reflection loss, etc. due to the light pipe. As a result, brilliance approximately equivalent to that of a background-art reflection type liquid-crystal display device can be achieved in a reflection mode. Moreover, brilliance not at all inferior to that of a background-art transmission type liquid-crystal display device can be achieved in a transmission mode. Leakage light is also slight, so that high contrast can be achieved. In addition, inversion of display does not occur between reflection and transmission, so that a liquid-crystal display device of a good visual recognition property can be obtained.
Moreover, a light path in the light pipe in the transmission mode can be elongated by provision of the light output means on the upper surface of the light pipe. Hence, the spread of light is widened, so that the intensity of a bright line can be relaxed. This effectively acts on both prevention of occurrence of moire and enhancement of uniformity of brightness advantageously, so that the reflection polarizer can be disposed on the lower surface of the light pipe so as to be brought into contact with and integrated with the light pipe through a tacky layer, or the like, easily. If such light output means is provided on the lower surface of the light pipe, it is necessary to provide a reflection polarizer separately in terms of the maintenance of the function of the light output means. As a result, the structure of the device is complicated because of increase in the number of parts and arrangement and fixation of the reflection polarizer.
Further, in the case of a light pipe having light output means constituted by slopes such as prism-like irregularities, the light pipe is excellent in directivity of reflected light through the slopes. Accordingly, light advantageous to visual recognition in a transmission mode can be formed efficiently, so that more brilliant display can be obtained. The light pipe is further excellent both in incident efficiency of external light and in transmission efficiency after reflection thereof. Accordingly, more brilliant display can be obtained also in a reflection mode by light emission excellent both in light utilizing efficiency and in uniformity. Further, generation of moire due to the foregoing directivity can be suppressed by oblique arrangement of the light output means, so that inhibition of visual recognition due to dazzling light can be prevented from being generated.
In a light pipe having scatter type light output means such as dots, embossed irregularities, or the like, used as the aforementioned light output means, output light in a transmission mode is output at a large angle of about 60 degrees. Accordingly, display becomes dark and hard to see in a frontal (vertical) direction. In addition, contrast is lowered by scattered light directly incident on the liquid-crystal cells. Accordingly, when a prism sheet is disposed for the purpose of controlling a light path, light is scattered in a reflection mode. As a result, a large part of light makes no contribution, so that display becomes very dark. Also in a reflection mode, external light is scattered by the light pipe, so that display becomes dark and hard to see because reflected light sufficient to be utilized for liquid-crystal display cannot be obtained through the reflection polarizer. Moreover, dots, or the like, are visually recognized too conspicuously. Accordingly, when a diffusion layer intensive in diffusing property is disposed as a countermeasure, both incident light in a reflection mode and reflected light thereof by the reflection polarizer are scattered to thereby make display dark.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.